NUTRITION AND FEEDING MECHANISMS 



223 



but the main function of the house and water current is feeding and not 

 locomotion. The house quickly becomes clogged with particles and is 

 abandoned in the matter of a few hours; a new one is constructed in 

 1 5-30 min. 



Ciliary Feeding by Larvae. Free-swimming planktonic larvae of many 

 animals feed by ciliary devices, often quite different from those of the 

 adults. AmOng such larvae are numbered the pilidium of nemertines, the 

 trochophore of annelids, veliger of molluscs and successive larval stages of 

 echinoderms. A veliger larva — that of the oyster for example — bears a 

 conspicuous ciliated disc or velum, which is used for feeding and locomo- 

 tion (Fig. 5.19(a)). Arising from the velum is a crown of large cilia which 



Fig. 5.19. Feeding Devices of Larvae 



(a). Veliger larva of the oyster. Large arrow above shows direction of movement; 

 smaller arrows in figure show direction of ciliary feeding currents; 1 : digestive diverti- 

 cula; 2: adductor muscle; 3: stomach; 4: style sac; 5: midgut; 6: rectum; 7: anus; 

 8 : mantle cavity ; 9 : oesophagus ; 1 : foot ; 1 1 : mouth ; 1 2 : ciliated tract at base of velum ; 

 13: velum (from Yonge, 1926). (6). Tornaria larva showing fields and furrows on sur- 

 face, and paths taken by feeding currents ; 1 : apical plate ; 2 : antero-dorsal sulculus ; 

 3: mid-dorsal field; 4: lateral sulcus; 5: locomotive girdle; 6: sub-dorsal sulculus; 

 7: lateral bay; 8: oral sulcus; 9: mouth; 10: pre-oral sulculus; 11 : pre-oral field (from 

 Garstang (30).) 



collect and throw particles on to a ciliated tract around the base, where they 

 become entangled in mucus and are carried back to the mouth (19). 



In echinoderm larvae, such as the auricularia and bipinnaria of star- 

 fishes, food particles are collected by longitudinal ciliated bands bordering 

 grooves or sulci, along which they are conveyed to the dorsal border of 

 the stomodaeum. The great extension of the longitudinal ciliated band 

 which results from folding and development of the larval arms greatly 

 increases the effective food-collecting area. 



The feeding process in tornaria larvae of enteropneusts is remarkably 

 similar to that of echinoderm larvae. The planktonic tornaria likewise 

 possesses longitudinal bands of cilia bordering a system of sulci which 

 lead to the mouth (Fig. 5.19(6)). The cilia beat transversely, sweeping 

 particles into the gutters, along which they are conveyed to the mouth 



